update how to apply style guide

as kind of a follow up to this and to setting up pre-commit ci, should there be a clean up of tox to remove recipes related to [style](https://github.com/nipy/nibabel/blob/1b9ed44d26c05f44e62bc7ce36f4b09dfda1803e/tox.ini#L138) (same for the [documentation](https://nipy.org/nibabel/devel/devguide.html#style-guide) and maybe [pyproject.toml](https://github.com/nipy/nibabel/blob/1b9ed44d26c05f44e62bc7ce36f4b09dfda1803e/pyproject.toml#L86)) _Originally posted by @Remi-Gau in https://github.com/nipy/nibabel/issues/1467#issuecomment-4002842672_